The present invention relates to electrophotographic image forming equipment for forming a latent image on a photoconductive element and developing it by a magnetic brush of dry two-component developer formed on a developing sleeve to which a bias for development is applied. More particularly, the present invention is concerned with a device for controlling the application of the bias to the developing sleeve.
Generally, in electrophotographic image forming equipment such as an electrophotographic copier, laser beam printer or facsimile machine, a latent image is electrostatically formed on the surface of a photoconductive element or image carrier and developed by a dry two-component developer which is a mixture of toner and is carrier particles. The developer is magnetically deposited on a developing roller or a developing sleeve to form a magnetic brush. It is a common practice with this kind of development to apply a bias for development to the developing roller or the developing sleeve. In an electrophotographic copier, for example, a motor for rotating the photoconductive element is energized on the operation of pressing copy button. Then, a charging unit for charging the surface of the photoconductive element and a bias power source are sequentially turned on in this order. As a result, the bias is applied to the developing roller or the developing sleeve, whereupon a predetermined copying operation begins. On the completion of the copying operation, the charging unit, bias power source and motor are sequentially turned off.
However, starting on a copying operation at the above-stated timing causes the carrier to deposit on a part of the photoconductive element coresponding to a leading or a trailing edge portion of a paper sheet in a several millimeters wide stripe configuration which extends crosswise on the element, as proved by experiments. Although the carrier so deposited on the photoconductive element does not directly effect the image transferred to a paper sheet, it is in due course scattered around in the copier to contaminate the latter. It is therefore necessary to prevent the carrier from depositing on the non-image area of the photoconductive element.